fix(stepper): error when selectedIndex is pre-set (#8035)

Fixes an error that is thrown if the `selectedIndex` is set before the steps are initialized.

Fixes #8031.

src/cdk/stepper/stepper.ts

@@ -149,13 +149,17 @@ export class CdkStepper {
   @Input()
   get selectedIndex() { return this._selectedIndex; }
   set selectedIndex(index: number) {
-    if (this._anyControlsInvalid(index)
-        || index < this._selectedIndex && !this._steps.toArray()[index].editable) {
-      // remove focus from clicked step header if the step is not able to be selected
-      this._stepHeader.toArray()[index].nativeElement.blur();
-    } else if (this._selectedIndex != index) {
-      this._emitStepperSelectionEvent(index);
-      this._focusIndex = this._selectedIndex;
+    if (this._steps) {
+      if (this._anyControlsInvalid(index) || index < this._selectedIndex &&
+          !this._steps.toArray()[index].editable) {
+        // remove focus from clicked step header if the step is not able to be selected
+        this._stepHeader.toArray()[index].nativeElement.blur();
+      } else if (this._selectedIndex != index) {
+        this._emitStepperSelectionEvent(index);
+        this._focusIndex = this._selectedIndex;
+      }
+    } else {
+      this._selectedIndex = this._focusIndex = index;
     }
   }
   private _selectedIndex: number = 0;
@@ -281,9 +285,12 @@ export class CdkStepper {
   }
 
   private _anyControlsInvalid(index: number): boolean {
-    this._steps.toArray()[this._selectedIndex].interacted = true;
+    const steps = this._steps.toArray();
+
+    steps[this._selectedIndex].interacted = true;
+
     if (this._linear && index >= 0) {
-      return this._steps.toArray().slice(0, index).some(step => step.stepControl.invalid);
+      return steps.slice(0, index).some(step => step.stepControl && step.stepControl.invalid);
     }
     return false;
   }

src/lib/stepper/stepper.spec.ts

@@ -21,6 +21,7 @@ describe('MatHorizontalStepper', () => {
       imports: [MatStepperModule, NoopAnimationsModule, ReactiveFormsModule],
       declarations: [
         SimpleMatHorizontalStepperApp,
+        SimplePreselectedMatHorizontalStepperApp,
         LinearMatHorizontalStepperApp
       ],
       providers: [
@@ -168,6 +169,18 @@ describe('MatHorizontalStepper', () => {
     it('should be able to move to next step even when invalid if current step is optional', () => {
       assertOptionalStepValidity(testComponent, fixture);
     });
+
+    it('should not throw when there is a pre-defined selectedIndex', () => {
+      fixture.destroy();
+
+      let preselectedFixture = TestBed.createComponent(SimplePreselectedMatHorizontalStepperApp);
+      let debugElement = preselectedFixture.debugElement;
+
+      expect(() => preselectedFixture.detectChanges()).not.toThrow();
+
+      let stepHeaders = debugElement.queryAll(By.css('.mat-horizontal-stepper-header'));
+      assertSelectionChangeOnHeaderClick(preselectedFixture, stepHeaders);
+    });
   });
 });
 
@@ -862,3 +875,16 @@ class LinearMatVerticalStepperApp {
     });
   }
 }
+
+@Component({
+  template: `
+    <mat-horizontal-stepper [linear]="true" [selectedIndex]="index">
+      <mat-step label="One"></mat-step>
+      <mat-step label="Two"></mat-step>
+      <mat-step label="Three"></mat-step>
+    </mat-horizontal-stepper>
+  `
+})
+class SimplePreselectedMatHorizontalStepperApp {
+  index = 0;
+}